I use PC’s, running XP Pro, with older quad core Q9300 CPU’s. I most often work with only one machine at a time as a stand-alone, using both simultaneously only when bouncing my VSTi machines virtual instruments audio out (as a stereo sub-mix) over into my main audio DAW machine, where I then work my audio tracks. I have a KVM switch too, 4-port for future expansion. I have lot’s of options to experiment with.
I’m actually planing on isolating ALL my PC’s away from my desk area, and I’d never track audio (my voice or an acoustic guitar) with more than one machine running.